Wind power plants with windmills operating with variable speed are only indirectly connected to the electricity network, which means that the generator has its own AC network where the frequency can vary. In order to transmit electricity to the electricity network, the AC current from the generator is converted into DC, and then to AC again, where the latter AC is adapted to the frequency of the electric network.
When transforming DC into AC, typically a pulse width modulated (PWM) frequency converter is applied, however resulting in uneven current having abrupt steps or discontinuities in voltage and amperage. In order to smooth this current for feeding into the network, AC filters with coils and condensers, so-called RCL filters, are used. In spite of the smoothing action of the filter, usually there will still be distortions of the AC in the network, depreciating the quality of the current. Furthermore, this smoothing provided by the filter implies consumption of power from the network to some extent. This is even more pronounced the more unstable the network receiving the current is functioning. Power absorption from the network during the smoothing results in reduced net transmission of power from the generator to the network. For the windmill owner, this entails reduced profits, which is a great disadvantage.